Pricey beef tenderloin might not be the first protein you think of when it comes to bulking up a cheese quesadilla. After all, this cut is most often reserved for filet mignon or chateaubriand. But when used smartly, a little tenderloin goes a long way. For this simple quesadilla, we seared the steak, then matched it with something saucy and spicy: a Venezuelan-style guasacaca sauce made from avocados, onion, vinegar, lime and jalapeños. The condiment is like a thin, tangy hybrid of salsa and guacamole, and is as fun to say as it is to eat.